An inductively coupled plasma optical emission spectrometer (ICP optical emission spectrometer) has been widely used in analysis of elements contained in a solution. Inductively coupled plasma has the advantages of not necessitating the use of electrodes exposed to plasma during the generation and having a smaller amount of contamination of impurities from the electrodes. On the other hand, apparatus for generating plasma other than the ICP optical emission spectrometer are subjected to contamination of impurities from electrodes or the like, so that the apparatus are not suitable for high-sensitivity elemental analysis.
In recent years, a microfluidic device, and a research field called μTAS, or lab-on-a-chip have been rapidly developed, in which a small flow channel, a reaction vessel, and an analytical device are built on a wafer by applying semiconductor processes, to try accomplishing a set of chemical experiments necessary for a blood test or the like on one chip. In this field, in order to perform high-sensitivity elemental analysis, a method of performing elemental analysis comprising the steps of generating microplasma, and introducing a nebulized solution therein has been developed.
As the microplasma, those obtained by miniaturizing a direct current plasma, a capacitively coupled plasma, an inductively coupled plasma, or the like has been known. For example, a microchemical analysis system for performing emission spectroscopic analysis (see, for example, Patent Publication 1) or the like has been proposed.
However, in order to generate stable plasma by these methods for generating plasma, there are some disadvantages in these methods that during the generation of plasma, a gas pressure reduction is necessitated, that a gas that can easily maintain plasma such as helium is used, or that radio frequency is necessitated. In addition, in order to prevent damages caused by heat to the apparatus while maintaining plasma, there are not only a disadvantage that a certain level of a gas flow rate is necessary so that its usability is poor in that a large tank is necessary or the like, but also a disadvantage that high electric power is necessitated for the generation of plasma. Furthermore, in order to introduce a sample into plasma, it is necessary to gasify the sample, so that a nebulizer is necessitated in order to spray the sample in a nebulized state. Since it is difficult to miniaturize the nebulizer, a considerable amount of flow rate of nebulized gas is necessitated. Therefore, when the nebulized gas is introduced into plasma, the nebulized gas perturbs the plasma. In order to tolerate the perturbation, the size of the plasma is necessitated to have a certain degree of largeness. Accordingly, it has been difficult to miniaturize an emission spectrometer in which a nebulizer is used, and no emission spectrometer having favorable performance has conventionally been obtained.
As an alternative method for generating plasma, there has been reported a method comprising inserting electrodes into a solution and conducting a direct current to the solution, thereby generating plasma (see, for example, Non-Patent Publication 1).
An advantage of this method is that a nebulizer is not necessitated because plasma is generated in a solution and the vaporization of the solution serves as a function of gasifying the sample. However, on the principle of the generation of plasma, the efficiencies for generating and maintaining plasma are higher on an interface between the surface of a solid electrode and a gas than those on a gas-liquid interface, in a solution, or in a gas. Therefore, there are some defects in the conventional method that since a solid electrode always contacts with the plasma, impurities contained in the solid electrode are vaporized, thereby making it difficult to avoid contamination of the impurities.
Patent Publication 1: Japanese Patent Laid-Open No. 2002-257785
Non-Patent Publication 1: Kazuhisa AZUMI, Masahiro SEO, and
Tadahiko MIZUNO; “Light Emission Spectroscopy from Various Metal Electrodes During Electrolysis), Electrochemistry, 67(4),1999, 349-354, internet <URL: 1111641682281—0.html>